What is LDP || Frequently asked Questions and Answers on MPLS LDP

What is LDP || Frequently asked Questions and Answers on MPLS LDP

1. Presented By - SIDDHARTH Interview Questions & Answers On MPLS-LDP

2. Question - What is LDP? LDP stands for Label Distribution Protocol. LDP is a protocol that automatically generates and exchanges labels between routers. Each router will locally generate labels for its prefixes and will then advertise the label values to its neighbors . LDP allows routers to establish label-switched paths (LSPs) through a network by mapping network-layer routing information directly to data link layer-switched paths. Answer Networking Study Guide

3. Question - What is the Multicast IP address used by LDP? LDP uses the multicast address 224.0.0.2 Answer Networking Study Guide

4. Question – Which source/destination port used for LDP neighbor establishment? LDP uses UDP Port 646 for neighbor establishment Answer Networking Study Guide

5. Question – What is LSR ID in LDP? Each router has a unique ID called the LSR (Label Switch Router) ID . By default it will select the highest IP address on a loopback interface. If you don’t have any loopback interfaces then we will use the highest IP address on a physical interface. Answer Networking Study Guide

6. Question - What are default timers of LDP? LDP Hello Interval – Default is 5 seconds. LDP Hold Timer - Default is 15 sec. If the two LDP peers have different LDP Hold times configured, the smaller of the two values is used as the Hold time for that LDP discovery source. Answer Networking Study Guide

7. Question – Explain LDP message types ? LDP uses the message types to establish and remove mappings and to report errors. All LDP messages have a common structure that uses a Type , Length , and Value (TLV ) encoding scheme . There are 4 types of LDP messages – 1. Discovery Messages 2. Session Messages 3. Advertisement Messages 4. Notification Messages Answer Networking Study Guide

8. Question – Explain LDP Discovery Message? Discovery messages announce and maintain the presence of a router in a network. Routers indicate their presence in a network by sending Hello messages periodically. LDP Uses Two Discovery Procedures – 1 . Basic Discovery - A router periodically sends LDP link hello messages through an interface. LDP link hello messages are sent as UDP packets addressed to the LDP discovery port. 2. Extended Discovery - LDP sessions between routers not directly connected are supported by LDP extended discovery. A router periodically sends LDP targeted hello messages to a specific address. Answer Networking Study Guide

9. Question – Explain LDP Session Messages ? Session messages establish, maintain, and terminate sessions between LDP peers . When a router establishes a session with another router learned through the hello message, it uses the LDP initialization procedure over TCP transport. When the initialization procedure is completed successfully, the two routers are LDP peers and can exchange advertisement messages. Answer Networking Study Guide

10. Question – Explain LDP Advertisement Messages Advertisement messages create, change, and delete label mappings for forwarding equivalence classes (FECs). Requesting a label or advertising a label mapping to a peer is a decision made by the local router. In general, the router requests a label mapping from a neighboring router when it needs one and advertises a label mapping to a neighboring router when it wants the neighbor to use a label. Answer Networking Study Guide

11. Question – Explain LDP Notification Messages Notification messages provide advisory information and signal error information. LDP sends notification messages to report errors and other events of interest.  There are two kinds of LDP notification messages : 1. Error notifications. 2. Advisory notifications. Answer Networking Study Guide

12. Question – Is LDP required for VPNV4 labels? No, because LDP is only responsible for the top most label is IGP label and MP- iBGP is responsible for vpnv4 label. Answer Networking Study Guide

13. Question – What is MPLS LDP Graceful Restart? LDP graceful restart enables a router whose LDP control plane is undergoing a restart to continue to forward traffic while recovering its state from neighboring routers. It also enables a router on which helper mode is enabled to assist a neighboring router that is attempting to restart LDP. Answer Networking Study Guide

14. Question - What is LDP Loop Detection? The LDP loop detection mechanism can detect looping LSPs and prevent LDP messages from looping forever. LDP loop detection can be in either of the following modes: 1 . Maximum hop count 2 . Path vector To implement loop detection in an MPLS domain, you must enable loop detection on every LSR in the MPLS domain Answer Networking Study Guide

15. Question – Explain LDP IGP Synchronization ? The MPLS LDP IGP Synchronization feature ensures that the Label Distribution Protocol (LDP) is fully established before the Interior Gateway Protocol (IGP) path is used for switching . If the IGP and LDP are not synchronized, packet loss can occur . LDP-IGP synchronization is supported only for directly connected peers and links with the platform label space. Answer Networking Study Guide

16. Question – Which authentication method used by LDP? LDP can uses Message Digest 5 (MD5) authentication method. Answer Networking Study Guide

17. Question - How can you reduce the number of label bindings on an LSR ? To reduce the number of label bindings on an LSR, you can either control the advertisement of label bindings via LDP in the outbound direction, or you can filter the incoming label bindings. Answer Networking Study Guide

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